First time I have heard anyone propose this, have honestly never seen it been played before in PTCGO either. That being said, I am definitely open to the inclusion of Alolan Vulpix and Baby Alolan Ninetales into Greninja builds. But, adding a Ninetales-GX strikes me as a bad idea due to a couple reasons:
On the other hand, the Baby Alolan Ninetales line combos well with Greninja at very little cost to the deck itself. That cost of course being adding a couple extra Pokemon might slow down the pace of the deck a bit, something you should be able to overcome with extra draw support. That being said, not only does Alolan Vulpix act as an early-game search card, but Baby Ninetales also acts as a vital wall against a format that solely relies on GXs in order to function. Unfortunately since this hasn't been tested yet of course, there is no guarantee that it will replace the Greninja/Talonflame archetype that has seen tournament level success this year. All in all, I don't believe Alolan Ninetales-GX has any place in a Greninja deck, but we might just see Baby Alolan Ninetales play alongside Greninja in the new format.
- For starters, Greninja prides itself on using one energy attacks, so attaching even two energies to attack disrupts the strategy, never mind three. The attacks Blizzard Edge and Ice Path GX simply require too much energy to use, so it doesn't work for a deck that only runs ten in total. Now I'm not saying there is no chance of success, but in my opinion to keep consistency which is something Greninja needs, stick to one energy attacks.
- Additionally, Alolan Ninetales-GX is a GX Pokemon which may seem like a good addition to a deck that doesn't utilize the new system, but there is one big flaw. One of Greninja's strongest points moving forward into the next rotation is that Choice Band is useless against Greninja because it normally doesn't run any GX or EX Pokemon. Therefore, when almost all decks run anywhere from one to three Choice Bands, you want to force dead cards into your opponent's hand at all times.
- Finally, Alolan Ninetales-GX doesn't have free retreat cost. I'm fully aware this is something you can't avoid, and Alolan Vulpix also suffers from not having free retreat cost, but it isn't something you can ignore. Giant Water Shuriken already discards energies, so losing them on retreating is always painful, especially for a deck like this.
The thing for me is very simple, Ninetales GX is not the only reason to include it. It's one of the few Pokemon that still allows Greninja's set up because of two very simple reasons:
1. Beacon lets you search out any Pokemon, from basic to stage 1,2 or BREAK. Greninja BREAK still needs this.
2. Unlike Ninetales GX (which is really optional, as described above) regular Alolan Ninetales is a wall for a metagame dominated by GX and EX. What this allows for is time and time is all Greninja BREAK needs because the moment your opponent is looking down multiple Greninja BREAKs while it still is trying to figure out an awnser to Alolan Ninetales is the moment you win.
The idea most certainly is there for the upcomming format and not the current one. However I believe that is what this topic is about. Feel free to correct me if Im reading BKT to BUS wrong in the title of the topic.
The set up with Alolan Vulpix is not only simple, gives you time and fits into the Water deck without doubt it also allows you to have a good early plan that is consistent. Something the current Greninja deck does not have. As before, Talonflame is good if it shows up as the opening Pokemon, which to date it cannot consistently do. Alolan Vulpix, Ninetales and GX on the other hand are not bound or limited in early game effectiveness.
Lastly if you feel that free retreats are an issue consider Manaphy EX as a tech option aswell. For sure you don't have to follow the Vulpix path but I know I would and will.
Cheers,